Flywheels have been used for many years for storing energy in a system, and then releasing that stored energy back into the system. They provide a smoothing effect for internal combustion engines and many kinds of power equipment. More recently, modern flywheel systems have become recognized as very attractive energy storage systems in electrical applications such as uninterruptible power supplies, utility load leveling, electric vehicles and battery replacement.
Modern flywheel systems used for storage of electrical energy, in the form of mechanical rotational inertial converted from electrical energy, include a flywheel hub and rim, and a rotor and stator on the hub shaft that function as an electric motor during storage of electrical energy and as a generator during regeneration of electrical energy when the stored energy is to be reapplied into the system. The flywheel system is normally contained in a vacuum enclosure that protects it from windage losses that would occur from operation in a gas atmosphere, and provides ballistic protection against catastrophic failure of a flywheel rotating at high speed.
The flywheel rim usually comprises a composite ring made of resin-impregnated filaments wound in the hoop direction. The rim can be made of one or multiple types of fiber in a concentric ring arrangement. Flywheel designs using multiple fibers typically use the lowest modulus fibers at the inside and the higher modulus fibers on the outside. One example would be using an E-glass/epoxy ring inside a carbon/epoxy ring. Placing the lower modulus fibers on the inside produces a more favorable stress distribution in the composite rim. The less stiff inner ring tends to grow radially under the centrifugal loading more than does the outer ring. Therefore, a component of radial compressive stress is generated as the inner ring grows into the outer ring. This compression tends to counteract the radial tensile stresses generated in isolated rings under centrifugal loading.
To minimize the cost of the complete rotor, it would be desirable to use the lowest cost fibers to make the composite rim. The current price per pound of E-glass fiber is roughly one tenth of the per pound price of standard modulus carbon fiber. Intermediate and high modulus carbon fibers increase significantly in price with elastic modulus to approximately five hundred times the per pound price of E-glass fiber. A rotor using mostly E-glass would be the most economical, but such a rotor would experience a large strain of its inner diameter when spinning due to the low elastic modulus and high strain to failure, approximately 2.5%. This large strain would make the development of a strain matching hub very difficult.
The prior methods for designing hubs that can handle both the large radial growth of the inside diameter of the composite rim and the high centrifugal loads generated from high speed rotation can be classified into two categories: strain matching and sliding joint hubs. Several schemes can be used to employ strain matching hubs with composite rim flywheels. One method is to use a simple metallic cylinder for a hub. This type of hub can handle the centrifugal forces from high speed rotation however the growth of the outer diameter is very small. Therefore, the composite rim must be made of high modulus carbon fiber or made of multiple intermediate modulus carbon fiber rings that are press-fit together. Press-fitting allows a much radially thicker rim to be used so that the interface diameter between the rim and hub can be made much smaller and the resulting hoop strain on the composite rim inner diameter is also much smaller. Both of these strain-matching methods are expensive either because of use of very expensive high modulus carbon fibers or because of excessive and inefficient use of intermediate modulus carbon fiber.
Another method for strain matching hubs is to use hubs with curved spokes. These spokes allow large deflections through bending and can match the strain on the inner diameter of composite rims made of both glass and carbon fiber combined. Many such designs have been previously designed and patented. The problem with these types of hubs is that they also require excess carbon fiber in the rim to limit the rim strain because the strain of the hub outer diameter, although greater than a solid cylinder, is still limited. These hubs also require complicated machining and can suffer from fatigue problems due to the high stresses in the areas where bending is occurring.
The second category of fly wheel hubs, those that use sliding joints, have been used in high speed rotating equipment either through the use of roll pins that are radially oriented or through spline connections. Both radial roll pins and straight sided splines allow a shaft or hub to transmit torque to and from the flywheel while eliminating radial stresses at the joint and accounting for a difference in growth due to spinning. Because pins or spline teeth are radially oriented, the flywheel is kept in the center location of the shaft/hub even when there is a difference in growth. This method works well when both the shaft/hub and the-flywheel are made of metal. However, a problem arises when using this type of hub mechanism for low cost filament wound composite rims: radial holes cannot be drilled into the composite for placement of pins because this would unacceptably weaken the rim. Likewise, spline teeth could not be cut into the composite rim, for the same reason.